A coalition of citizens and government agencies has unveiled a mightily ambitious master plan by an eminent design firm that would transform the threadbare path under the Metrorail along South Dixie Highway into a garland of verdant gardens, playgrounds and recreational spots linked by 10 miles of side-by-side bike and pedestrian trails.

The vision is expansive: From the downtown lip of the Miami River, where the plan calls for an outdoor viewing “room” composed of cascading stone steps, to suburban Dadeland, where it contemplates workout stations and sports courts cradled between Metrorail’s two southernmost stations, the proposed Underline would embed alternative transportation within a long, slender park to create a potentially defining urban amenity.

But it’s also expensive: Estimates put the cost of executing the full master plan by James Corner Field Operations, co-designers of Manhattan’s uber-successful High Line, at $120 million. That’s due in part to the complexity of ensuring safety along the pathway’s 36 roadway intersections, which include some of the busiest — and most perilous — in the city. The plan calls for gently sloping bike and pedestrian bridges over five of those intersections and at least minor modifications to all the rest, at a cost of about $20 million.

The plan also envisions reestablishment along the Underline of some mostly vanished native habitats, including the pine rocklands and hardwood hammocks that once covered the area, though in a pared-down version that could thrive in the intensely urban environment.

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The release of the 400-page plan represents a big milestone for a project that 18 months ago was no more than a figment of the imagination of a handful of bike and parks advocates. The proposal has since won the active support of Miami-Dade Mayor Carlos Gimenez, the county’s transit and parks agencies, and the cities of Miami, Coral Gables and South Miami, which the Underline would traverse.

Backers say they’re determined to make sure the Underline doesn’t remain just pretty pixels in a digital file. They plan to use the $5 million in government grants so far in hand to begin work next year on the project’s first “demonstration” phase, a three-quarter-mile-long stretch around the Brickell station that would be laden with distinctive features, including the so-called River Room on the riverbank.

TheBrickell Backyard segment would also feature “picnic pockets,” a dog park, an outdoor gym and an Oolite Room that have a play area and a climbing wall built around an existing natural rock outcropping. Just south of that, the Brickell Hammock Trail section would take up a broad swath of green under the Metrorail adjacent to the city’s Simpson Park, one of the few surviving bits of the area’s original hammock. A replenished canopy of oaks and other hardwood trees would shade a playground.

We want to show that we’re serious, but we also we hope to have enough built so that it’s meaningful.

Meg Daly

Those features, like much of the rest of the Underline plan, answer to a wish list compiled in a series of public meetings with residents, Underline backers say. They’re now trying to figure out how much of the Brickell-area plan can be realized for $5 million. Though the state Legislature appropriated an additional $3 million for the project, the money was vetoed by Gov. Rick Scott.

“We want to show that we’re serious, but we also we hope to have enough built so that it’s meaningful,” said Meg Daly, who originated the idea and chairs Friends of the Underline, the nonprofit group formed to guide its realization.

The plan, which has been vetted by several agencies, was formally presented Wednesday evening to Underline supporters, including some corporate representatives whose financial support may prove critical to the Underline plan’s success.

On Monday, Friends of the Underline will repeat the presentation to the public at the University of Miami. Daly encouraged attendance, noting it would be the last chance for people to comment on the master plan, which is meant to guide development of the project as it proceeds in phases over several years. The plan then goes to the Miami-Dade County Commission for approval.

The Brickell section is one of two proposed demonstration projects designed in detail by Field Operations under its $500,000 contract, underwritten by foundations and the three contiguous cities. Next to be built would be a one-mile section stretching from the entrance to the University of Miami to Red Road. It would feature a sunken rain garden, taking advantage of a natural depression in the ground, a skateboard rink that could convert into an event space after dark, and a University Metrorail station that would be enlivened by food kiosks powered by solar panels.

Supporters plan to return to the Legislature and hope for support from Scott. But they’ve also formed a committee to garner corporate support and sponsorships. The idea, Daly said, is for the estimated $20 million needed to build out all planned park spaces to be raised through sponsorships and donations.

Because the Underline is also expected to boost businesses along the pathway and help attract mixed-use development to Metrorail stations — a key goal of the county — supporters will also explore ways to draw on the additional tax revenue thus generated to underwrite construction, Daly said, noting that the High Line has been credited with stimulating billions of dollars worth of development around it. A study of the Underline’s potential economic impact is now under way.

The swelling interest in the Underline comes as local governments, business groups and employers are pressing for development of alternatives to cars amid worsening traffic woes and a downtown revival fed by two successive development booms. The Underline, supporters say, would be a multi-purpose route linking bike commuters, casual riders and pedestrians not just to recreational spots, workplaces or shopping and dining close to home, but also to the spine of the county’s rail-transit system, which now includes a spur to Miami International Airport.

A parallel effort is now under way along the privately owned Ludlam Trail — like the Metrorail line once a corridor for the Florida East Coast railroad. The county’s planning board on Monday endorsed a plan that would allow development by FEC at four major intersections while turning most of the 6-mile corridor, which parallels Ludlam Road, into a walking and cycling trail and linear park. The Ludlam and Underline trails would connect at Dadeland.